An Improved Framework for Estimating Organic Carbon Content of Mangrove Soils Using loss-on-ignition and Coastal Environmental Setting.

The use of loss on ignition (LOI) measurements of soil organic matter (SOM) to estimate soil organic carbon (OC) content is a decades-old practice. While there are limitations and uncertainties to this approach, it continues to be necessary for many coastal wetlands researchers and conservation practitioners without access to an elemental analyzer. Multiple measurement, reporting, and verification (MRV) standards recognize the need (and uncertainty) for using this method. However, no framework exists to explain the substantial differences among equations that relate SOM to OC; consequently, equation selection can be a haphazard process leading to widely divergent and inaccurate estimates. To address this lack of clarity, we used a dataset of 1,246 soil samples from 17 mangrove regions in North, Central, and South America, and calculated SOM to OC conversion equations for six unique types of coastal environmental setting. A framework is provided for understanding differences and selecting an equation based on a study region's SOM content and whether mineral sediments are primarily terrigenous or carbonate in origin. This approach identifies the positive dependence of conversion equation slopes on regional mean SOM content and indicates a distinction between carbonate settings with mean (± 1 S.E.) OC:SOM of 0.47 (0.002) and terrigenous settings with mean OC:SOM of 0.32 (0.018). This framework, focusing on unique coastal environmental settings, is a reminder of the global variability in mangrove soil OC content and encourages continued investigation of broadscale factors that contribute to soil formation and change in blue carbon settings. Supplementary Information: The online version contains supplementary material available at 10.1007/s13157-023-01698-z.

Utilizing water level draw-down to remove excess organic matter in a constructed treatment wetland.

Constructed treatment wetlands are commonly used to enhance surface water nutrient removal following traditional wastewater treatment. However, the constant inflow may necessitate continuous wetland inundation, leading to persistent anaerobic conditions and the accumulation of organic matter (OM) as suspended detrital flocculent (floc) and soil OM. This study investigated if temporary water level draw-down (WLDD) could promote OM consolidation and oxidation without impacting nutrient removal efficiency. A large-scale, 2-y, before-after-control-impact field experiment at the Orlando Easterly Wetland (Christmas, FL, USA) was complemented by an intact soil core laboratory experiment with varied WLDD regimes. Changes in floc thickness, soil elevation, and surface water and soil nutrients were quantified. Field experiment results demonstrated an average floc thickness reduction of 60 % and soil elevation decline of 2.7 cm persisted after return to normal flow operation. This reduction was achieved with one ∼3-week dry event for two consecutive years and removed an estimated 7.5 years' worth of accumulated floc. Intact soil core results showed a direct relationship (R2 = 0.93) between days of WLDD and cumulative CO2-C loss, despite oxidation only accounting for 4-5 % of OM loss (and consolidation accounting for the remaining 95-96 %). While soil nitrogen (N) and phosphorus (P) concentrations did tend to increase during WLDD, outflow surface water N was not affected by the WLDD. Soluble reactive P increased for ∼36 days following reflooding, then returned to baseline. Incorporating WLDD into wetland management every few years could significantly reduce the frequency of costly cell renovation projects aimed at removing accumulated OM.

Coastal riverine wetland biogeochemistry follows soil organic matter distribution along a marsh-to-mangrove gradient (Florida, USA).

Many subtropical coastal wetland vegetation communities are transitioning from herbaceous marsh to woody mangrove, often facilitated by sea-level rise. This study investigated the relationships between vegetation community (upstream marsh, ecotone/transition, and downstream mangrove), salinity (S), and soil biogeochemistry in wetlands along three rivers on the Florida Gulf coast (the Little Manatee, Peace, and Fakahatchee Rivers). Vegetation was surveyed, and soil and water samples were collected during both the dry and the wet season and analyzed for biogeochemical properties (soil: bulk density, pH, organic matter, extractable inorganic and total nutrients, dissolved organic carbon (DOC), and microbial biomass carbon; water: inorganic nutrients and DOC) and processes (greenhouse gas production) while salinity and water level were continuously monitored in the field. Results indicated landscape-scale patterns in soil biogeochemistry differed significantly by river and were most strongly correlated with soil organic matter content, regardless of vegetation community or salinity regime. Contrary to expectations, soil organic matter content gradients were not always inversely related to salinity gradients, and methane production was observed in moderate- (S = 12) and high- (S = 34) salinity mangrove communities. The vegetation ecotone experienced seasonally variable salinity and did not serve as a true biogeochemical intermediate between the marsh and mangrove communities. This study demonstrates the need for site-specific studies of biogeochemical gradients in coastal wetlands and indicates the marsh-to-mangrove ecotone is not a proxy for salinity or biogeochemical tipping points. Instead, soil organic matter content is suggested as the most relevant indicator of biogeochemical properties and processes in wetlands along coastal rivers, superseding vegetation community or salinity.

Characterization of herbaceous encroachment on soil biogeochemical cycling within a coastal marsh.

Vegetation transitions occur globally, altering ecosystem processing of organic matter and changing rates of soil biogeochemical cycling. In coastal marshes, more salt- and inundation-tolerant herbaceous species are encroaching on less tolerant species, concomitant with sea level rise. These species shifts could disrupt ecosystem services such as soil organic matter storage and the cycling of carbon (C), nitrogen (N), and phosphorus (P). To determine how these ecosystem processes were affected by encroachment, we characterized biogeochemical properties and functions along a transect of encroaching Distichlis spicata L. Greene (saltgrass) on Spartina bakeri Merr. (cordgrass), two herbaceous species. During both the wet and dry season, nine soil cores were obtained from three community types: saltgrass end member, transition zone, and cordgrass end member. Total soil C, N, and organic matter were greatest within the saltgrass and transition zones. The saltgrass and transition zone soils also supported higher rates of enzyme activity and potentially mineralizable N and P than cordgrass soils during the dry season, and greater potential CO2 production and microbial biomass C during the wet season. Generally, the transition zone functioned similarly to the saltgrass zone and the encroachment gradient coincided with a 33 cm elevation change. Seasonally, low extractable nutrient availability (nitrate and soluble reactive phosphorus) during the dry season was correlated with overall greater enzyme activity (N-acetyl-ß-D-glucosidase, alkaline phosphatase, ß-glucosidase, xylosidase, and cellobiosidase) and potentially mineralizable N and phosphorus (P) rates. This study demonstrates that shifts in dominant herbaceous species and accompanying abiotic gradients alters biogeochemical processing of organic matter within coastal marshes.

Characterization of coastal wetland soil organic matter: Implications for wetland submergence.

High rates of relative sea level rise can cause coastal wetland submergence, jeopardizing the stability of soil organic matter (SOM) sequestered within wetlands. Following submergence, SOM can be lost through mineralization, exported into the coastal ocean, or reburied within adjacent subtidal sediments. By combining measures of soil physicochemical properties, microbial community abundance, organic carbon fractionation, and stable isotope signatures, this study characterized subsurface SOM within a coastal wetland to inform its potential fate under altered environmental conditions. Nine soil cores were collected to a depth of 150 cm from a wetland currently experiencing rapid erosion and submergence within Barataria Bay, LA (USA), and were sectioned into 10 cm intervals. Each soil segment was analyzed to determine total carbon (C), nitrogen (N), phosphorus (P), and stable isotope (δ13C and δ15N) content, as well as extractable ammonium (NH4+), nitrate (NO3-), and soluble reactive phosphorus (SRP). Extractable NH4+ and SRP concentrations increased 7× and 11×, respectively, between 0-10 cm and 130-140 cm. Through quantitative PCR, number of gene copies of bacteria and sulfate reduction genes were found to decrease with depth while there was no change in number of gene copies of archaea. This study also demonstrated only small decreases in labile: refractory C ratios with depth; by combining δ15N data with labile:refractory C ratios and no observed change in C:N ratios with depth, we inferred the presence of minimally processed organic material within deep soils and high nutrient availability, challenging the applicability of the traditional theory of selective preservation and decreased soil quality with depth. As wetland submergence progresses and soils are exposed to oxygenated seawater, this relatively labile SOM and bioavailable N and P stored at depth has the potential for rapid mineralization and/or export into the coastal zone.

Toward a mechanistic understanding of "peat collapse" and its potential contribution to coastal wetland loss.

Coastal wetlands are susceptible to loss in both health and extent via stressors associated with global climate change and anthropogenic disturbance. Peat collapse may represent an additional phenomenon contributing to coastal wetland loss in organic-rich soils through rapid vertical elevation decline. However, the term "peat collapse" has been inconsistently used in the literature, leading to ambiguities regarding the mechanisms, timing, and spatial extent of its contribution to coastal wetland loss. For example, it is unclear whether peat collapse is distinct from general subsidence, or what biogeochemical changes or sequence of events may constitute peat collapse. A critical analysis of peer-reviewed literature related to peat collapse was supplemented with fundamental principles of soil physics and biogeochemistry to develop a conceptual framework for coastal wetland peat collapse. We propose that coastal wetland peat collapse is a specific type of shallow subsidence unique to highly organic soils in which a loss of soil strength and structural integrity contributes to a decline in elevation, over the course of a few months to a few years, below the lower limit for emergent plant growth and natural recovery. We further posit that coastal wetland peat collapse is driven by severe stress or death of the vegetation, which compromises the supportive structure roots provide to low-density organic soils and shifts the carbon balance of the ecosystem toward a net source, as mineralization is no longer offset by sequestration. Under these conditions, four mechanisms may contribute to peat collapse: (1) compression of gas-filled pore spaces within the soil during dry-down conditions; (2) deconsolidation of excessively waterlogged peat, followed by transport; (3) compaction of aerenchyma tissue in wetland plant roots, and possibly collapse of root channels; and (4) acceleration of soil mineralization due to the addition of labile carbon (dying roots), oxygen (decreased flooding), nutrients (eutrophication), or sulfate (saltwater intrusion). Scientists and land managers should focus efforts on monitoring vegetation health across the coastal landscape as an indicator for peat collapse vulnerability and move toward codifying the term "peat collapse" in the scientific literature. Once clarified, the contribution of peat collapse to coastal wetland loss can be evaluated.

Sampling, Sorting, and Characterizing Microplastics in Aquatic Environments with High Suspended Sediment Loads and Large Floating Debris.

The ubiquitous presence of plastic debris in the ocean is widely recognized by the public, scientific communities, and government agencies. However, only recently have microplastics in freshwater systems, such as rivers and lakes, been quantified. Microplastic sampling at the surface usually consists of deploying drift nets behind either a stationary or moving boat, which limits the sampling to environments with low levels of suspended sediments and floating or submerged debris. Previous studies that employed drift nets to collect microplastic debris typically used nets with ≥300 µm mesh size, allowing plastic debris (particles and fibers) below this size to pass through the net and elude quantification. The protocol detailed here enables: 1) sample collection in environments with high suspended loads and floating or submerged debris and 2) the capture and quantification of microplastic particles and fibers <300 µm. Water samples were collected using a peristaltic pump in low-density polyethylene (PE) containers to be stored before filtering and analysis in the lab. Filtration was done with a custom-made microplastic filtration device containing detachable union joints that housed nylon mesh sieves and mixed cellulose ester membrane filters. Mesh sieves and membrane filters were examined with a stereomicroscope to quantify and separate microplastic particulates and fibers. These materials were then examined using a micro-attenuated total reflectance Fourier transform infrared spectrometer (micro ATR-FTIR) to determine microplastic polymer type. Recovery was measured by spiking samples using blue PE particulates and green nylon fibers; percent recovery was determined to be 100% for particulates and 92% for fibers. This protocol will guide similar studies on microplastics in high velocity rivers with high concentrations of sediment. With simple modifications to the peristaltic pump and filtration device, users can collect and analyze various sample volumes and particulate sizes.

Characterizing nutrient distributions and fluxes in a eutrophic reservoir, Midwestern United States.

Harmful algal blooms are increasingly common in aquatic ecosystems and have been linked to runoff from agricultural land. This study investigated the internal nutrient (i.e., phosphorus (P) and nitrogen (N)) dynamics of a eutrophic reservoir in the Midwestern United States to constrain the potential for sedimentary nutrients to stimulate harmful algal blooms. The spatial distribution of nutrients in the water column (soluble reactive P (SRP), nitrate/nitrite-N (NOx-N), and ammonium-N (NH4+-N)) and sediments (total P, total carbon (C), total N, and organic matter (OM)) were quantified and mapped. Water column nutrients varied spatially and temporally, with generally higher concentrations near the dam wall during normal lake levels. The upper portion of the lake, near the inlet, was sampled during a flood event and had overall higher nutrient concentrations and lower chlorophyll levels compared to normal lake level samples. Mean sedimentary total P (936mg/kg) was ~30% higher in the reservoir than the surrounding upland soils, with the highest concentrations near the dam wall (1661mg/kg) and a significant positive correlation found between sedimentary total P, total C, and OM. Additionally, 15 intact sediment cores were manipulated ex situ to examine mechanisms of nutrient flux across the sediment-water interface (SWI) that may trigger algal blooms. Core treatment conditions included advection (i.e., simulating potential nutrient fluxes during wind events through sediment resuspension) and diffusion. Core experiments indicated both advective and diffusive conditions at the SWI may trigger the flux of nutrients important for algal growth from lake sediments, with diffusion contributing both N and P to the water column, while intense advection increased water column N, but decreased P. Release of P to the water column may be more diffusion-driven than advection-driven, whereas N release to the water column appears to be both diffusion- and advection-driven.